An MRAM is a promising nonvolatile memory from a viewpoint of high integration and high-speed operation. In the MRAM, a magnetoresistance element that exhibits a “magnetoresistance effect” such as TMR (Tunnel MagnetoResistance) effect is utilized. In the magnetoresistance element, for example, a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include a magnetization fixed layer (pinned layer) whose magnetization direction is fixed and a magnetization free layer (free layer) whose magnetization direction is reversible.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization directions of the pinned layer and the free layer are “anti-parallel” to each other becomes larger than a resistance value (R) when the magnetization directions are “parallel” to each other due to the magnetoresistance effect. The MRAM uses the magnetoresistance element having the MTJ as a memory cell and nonvolatilely stores data by utilizing the change in the resistance value. For example, the anti-parallel state is related to data “1” and the parallel state is related to data “0”. Data writing to the memory cell is performed by switching the magnetization direction of the free layer.
Conventionally known methods of data writing to the MRAM include an “asteroid method” and a “toggle method”. According to these write methods, a magnetic switching field necessary for switching the magnetization direction of the free layer increases in substantially inverse proportion to a size of the memory cell. That is to say, a write current tends to increase with the miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write current with the miniaturization, there is proposed a “spin transfer method” (for example, refer to Japanese Patent Publication JP-2005-093488A, and J. C. Slonczewski, Current-driven excitation of magnetic multilayers, Journal of Magnetism and Magnetic Materials, 159, L1-L7, 1996). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched (hereinafter referred to as “Spin Transfer Magnetization Switching”).
U.S. Pat. No. 6,834,005 discloses a magnetic shift resister that utilizes the spin transfer. The magnetic shift resister stores data by utilizing a domain wall in a magnetic body. In the magnetic body having a large number of separated regions (magnetic domains), a current is so supplied as to pass through the domain wall and the current causes the domain wall to move. The magnetization direction in each of the regions is treated as a record data. For example, such a magnetic shift resister is used for recording large quantities of serial data. It should be noted that the domain wall motion in a magnetic body is reported also in Yamaguchi et al., Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires, PRL, Vol. 92, pp. 077205-1-4, 2004.
A “domain wall motion type MRAM” that utilizes the current-driven domain wall motion is described in Japanese Patent Publication JP-2005-191032A and International Publication WO 2007/020823. According to the MRAM described in these documents, a magnetization fixed layer whose magnetization direction is fixed and a domain wall motion layer connected to the magnetization fixed layer through a tunnel insulating layer form an MTJ. The domain wall moves in the domain wall motion layer, and a resistance value of the MTJ, namely a memory data varies depending on a location of the domain wall. Moreover, the domain wall motion layer includes not only a domain wall motion section whose magnetization direction is reversible but also a section whose magnetization direction does not substantially change. In this sense, the domain wall motion layer is hereinafter referred to not as a magnetization free layer but as a “magnetic recording layer”.
FIG. 1 shows a structure of a magnetic recording layer 100 described in Japanese Patent Publication JP-2005-191032A. The magnetic recording layer 100 has a linear shape. More specifically, the magnetic recording layer 100 has a connector section 103 overlapping with the tunnel insulating layer and the magnetization fixed layer, constricted sections 104 adjacent to both ends of the connector section 103, and a pair of magnetization fixed sections 101 and 102 respectively formed adjacent to the constricted sections 104. The magnetization fixed sections 101 and 102 are respectively provided with fixed magnetizations whose directions are opposite to each other. Furthermore, write terminals 105 and 106 are electrically connected to the magnetization fixed sections 101 and 102, respectively. By using the write terminals 105 and 106, a write current penetrating through the connector section 103, the pair of constricted sections 104 and the pair of magnetization fixed sections 101 and 102 in the magnetic recording layer 100 can be supplied.
FIG. 2 shows a structure of a magnetic recording layer 110 described in International Publication WO/2007/020823. The magnetic recording layer 110 has a U-shape. More specifically, the magnetic recording layer 110 has a first magnetization fixed region 111, a second magnetization fixed region 112 and a magnetization switching region 113. The magnetization switching region 113 overlaps with a magnetization fixed layer 130. The magnetization fixed regions 111 and 112 are so formed as to extend in a Y direction, and the magnetization directions thereof are fixed in the same direction. On the other hand, the magnetization switching region 113 is so formed as to extend in a X direction and has reversible magnetization. Therefore, a domain wall is formed at a boundary B1 between the first magnetization fixed region 111 and the magnetization switching region 113 or at a boundary B2 between the second magnetization fixed region 112 and the magnetization switching region 113. Moreover, the magnetization fixed regions 111 and 112 are connected to current supply terminals 115 and 116, respectively. By using these current supply terminals 115 and 116, it is possible to flow a write current in the magnetic recording layer 110. The domain wall moves within the magnetization switching region 113 depending on a direction of the write current. The magnetization direction of the magnetization switching region 113 can be controlled by the domain wall motion.